This invention is directed to a locking mechanism for use with a metering product of the type including value rods mounted to the rotatable drive shaft which rotates the print head. In particular, it is directed to a locking device which allows or prevents the axial movement of the value rods according to the rotary position of the drive shaft.
Postage meters are used to dispense postage in lieu of postage stamps. They do so by printing the postage value directly on the letter or on a separate slip of paper which is secured to the letter or package by an adhesive. Meters are constructed with many anti-fraud devices and features to prevent improper use or tampering. Some of the anti-fraud devices prevent access to critical components while others substantially prevent unauthorized changes to critical components. Some components are made so that unauthorized tampering or use will be evident upon inspection of the meter. One such meter is shown in U.S. Pat. No. 4,644,142, issued Feb. 17, 1987.
One critical component is that used to print the postage value. The printing elements are often print wheels which are mounted adjacent an opening in the indicium plate mounted to the print head. The print head rotates one complete revolution to make one impression. While the print head is at the home (stationary) position, the rotary positions of the print wheels, and thus the values of the impressions printed, can be changed. This can be accomplished through a drive train including elongate rods, called value rods, mounted to the drive shaft. The value rods rotate with the drive shaft, which rotates the print head, and move axially to change the rotary positions of the print wheels. The print wheels are protected from tampering while the drive shaft and print head are in the home position by non-removable shielding. However, the print head is necessarily exposed during that portion of the print cycle surrounding the making of the impression and the inking of the die. Tampering is possible by halting the rotation of the print head during this portion of the print cycle unless some form of protection is provided.